This invention is directed to a system particularly useful for measuring and indicating the state of charge of a storage battery. The invention is especially useful for monitoring rechargeable storage batteries such as those used in battery powered vehicles which may include various battery powered tools, such as fork lifts or the like, and it will be described in detail in this context. However, the inventive system may be used with any battery powered system using rechargeable or non-rechargeable batteries.
Circuitry is provided for integrating a signal related to fluctuations in the battery terminal voltage and for displaying the state of charge of the battery in terms of percentage charge remaining in the battery. The display is similar to a display showing the fuel remaining in a conventional gasoline powered vehicle and is therefore quite easy for an operator familar only with gasoline powered vehicles to read and understand. The system may also be provided with a deep discharge detector which, when the remaining charge in the battery has been depleted below a predetermined level, disables the various tools on the vehicle, leaving only those systems that are essential for the operator to be able to return to a battery charging station.
The system may be fabricated using any device which is capable of measuring and indicating the integral of an electrical signal. Such devices include, for example, electronic devices such as counters, electromechanical devices such as stepper motors, and electrochemical devices such as coulometers, and the inventive system will be described in circuits employing some such devices. It is, however, contemplated that the inventive system may be used advantageously with any integrating device.
In the preferred embodiment, connection of the battery to the vehicle results in the actuation of a threshold circuit which detects whether the voltage present at the terminals of the battery is above an upper threshold value. Insofar as a newly charged battery has an output voltage which is significantly higher than the nominal terminal voltage, the upper threshold value is picked to be about 10 percent above the nominal terminal voltage. If this threshold voltage is detected by the circuit, it is likely that the battery is freshly charged; and the threshold circuit causes the state of charge monitoring circuitry to produce an indication the battery is fully charged.
As the battery is used, varying load conditions placed across the battery cause the voltage to be reduced. In the preferred embodiment, the magnitude and duration of each of these voltage reduction is monitored by a second threshold circuit which produces an output whenever the terminal voltage falls below a lower threshold value. The output of this threshold circuit is connected to circuitry which generates a train of pulses in response to reductions in voltage. The number of pulses generated is a function of the time during which the terminal voltage is below the lower threshold voltage. Illustratively, the pulse generating circuitry takes the form of either a voltage controlled oscillator or a relaxation oscillator.
The pulse generating circuitry is in turn connected to integrating means, preferably an electronic counter, for counting the pulses and accumulating the count, thus generating an integral which is proportional to the total time that the terminal voltage is below the lower threshold voltage. In typical operation, this integration is performed over a hundred or more separate cycles in which the battery terminal voltage falls below the lower threshold level and then recovers to a level greater than said threshold.
The output of the integrating means furnishes an indication of the state of charge. This indication is more accurate than prior art devices since the integrating means accumulates the count for each time the terminal voltage falls below the lower threshold value. The output of the electronic counter may conveniently be read by converting it to an analogue signal which is used to drive a conventional d' Arsonval electric meter. The output of the integrating means may also activate an alarm which warns the operator that the state of charge of his vehicle's battery is at a predetermined low state of charge and, at a lower level of charge, may disable auxiliary functions on the vehicle such as the fork lift, thereby forcing the operator to return to the base station for a fresh battery.
Alternatively, the integrating means can be used with a threshold circuit that produces a single pulse each time the battery terminal voltage falls below the lower threshold level. Although not as accurate as the preferred embodiment, accumulation of said pulses over several such reductions below threshold is a crude measure of the state of charge of the battery.
Other alternatives include the use of a plurality of threshold means connected in series to produce a signal, for example, only when the battery terminal voltage falls below a threshold level and remains below such level for a specified period of time.
In another embodiment the output of the threshold circuit is integrated directly by a suitable integrator such as a coulometer. Illustratively, the magnitude and duration of voltage reductions caused by varying load conditions placed across the battery are monitored by a multiple-threshold circuit whose output signal is related to the magnitude and duration of the voltage reductions. This signal is stored by a integrator which drives a display and may also activate an alarm as in the first embodiment. Again, the output of the integrator furnishes an indication of the state of charge.